In oil and gas wells, it often becomes necessary to stimulate hydrocarbon flow in order to attain economical feasible production rates, or to increase production rates. The technique frequently used to stimulate wells in such a manner is termed “fracturing”, and refers to a method of pumping a fluid into the well until the pressure increases to a level sufficient to fracture the subterranean geological formation, resulting in cracks in the formation. These cracks are capable of carrying product to the well bore at a significantly higher flow rate.
In general, proppants are extremely useful to keep open fractures imposed by hydraulic fracturing of a subterranean formation, e.g., an oil or gas bearing strata. Typically, the fracturing is desired in the subterranean formation to increase oil or gas production. As noted above, fracturing is caused by injecting a viscous fracturing fluid, foam, or other suitable fluid at high pressure into the well to form fractures. As the fracture is formed, a particulate material, referred to as a “propping agent” or “proppant” is placed in the formation to maintain the fracture in a propped condition when the injection pressure is released. As the fracture forms, the proppants are carried into the well by suspending them in additional fluid or foam to fill the fracture with a slurry of proppant in the fluid or foam. Upon release of the pressure, the proppants form a “pack” which serves to hold open the fractures. The goal of using proppants is to increase production of oil and/or gas by providing a highly conductive channel in the formation. Choosing the correct proppant, therefore, is critical to the success of well stimulation.
The propped fracture thus provides a highly conductive channel in the formation. The degree of stimulation afforded by the hydraulic fracture treatment is largely dependent upon formation parameters, the fracture's permeability and the fracture's propped width. If the proppant is an uncoated substrate, e.g., sand, and is subjected to high stresses existing in a gas/oil well, the substrate may be crushed to produce ‘fines’ (particles with a sub-100 mesh (˜120 micron) size) of crushed proppant. Fines will subsequently reduce conductivity within the proppant pack. However, a resin coating will enhance crush resistance of a coated particle above that of the substrate alone and prevent crushed fine particles from migrating back to the wellbore area or plugging the remaining proppant pack.
Glass beads had been used as propping materials (see U.S. Pat. No. 4,068,718, for example). Their disadvantages include the high costs of energy and production, as before, and their severe drop in permeability at elevated pressures (above about 35 MPa) because of their excessive crushing at downhole conditions. Thus, the use of glass beads alone is not currently favored in the field. Rather, three different types of propping materials, i.e., proppants, are currently employed.
The first type of proppant is a sintered ceramic granulation/particle, usually aluminum oxide, silica, or bauxite, often with clay-like binders or with incorporated hard substances such as silicon carbide (e.g., U.S. Pat. No. 4,977,116 to Rumpf et al, incorporated herein by reference, EP 0 087 852, EP 0 102 761, or EP 0 207 668). The ceramic particles have the disadvantage than the sintering must be done at high temperatures, resulting in high-energy costs. In addition, expensive raw materials are used. They have relatively high bulk density, and often have properties similar to those of corundum grinding materials, which causes high wear in the pumps and lines used to introduce them into the drill hole. Also, during production after the hydraulic frac job, the abrasive particles that flow back may cause severe wear on valves and chokes at the wellhead.
The second type of proppant is made up of a large group of known propping materials from natural, relatively coarse, sands, the particles of which are roughly spherical such that they can allow significant flow. Exemplary proppants of this type are described in U.S. Pat. No. 5,188,175.
The third type of proppant, and that which is related to the coated resins of the present invention, includes proppants of type one and two above that are coated with a layer of synthetic resin such as described in U.S. Pat. No. 5,420,174 to Deprawshad et al; U.S. Pat. No. 5,218,038 to Johnson et al; and U.S. Pat. No. 5,639,806 to Johnson et al.
Known resins used in resin coated proppants include epoxy, furan, phenolic resins, resole-modified Novolac resins, and combinations of these resins. The resins are typically from about 1 to about 8 percent by weight of the total coated particle. The particulate substrates for resin coated proppants are typically described to be sand, ceramics or other particulate substrate and typically have a particle size in the range of USA Standard Testing screen numbers from about 8 to about 100 (i.e. screen openings of about 0.0937 inch to about 0.0059 inch).
Resin coated proppants can be further classified into precured and curable resin coated proppants. Precured resin coated proppants comprise a substrate coated with a resin which has been significantly crosslinked. The resin coating of the precured proppants provides crush resistance to the substrate. Since the resin coating is already cured before it is introduced into the well, even under high pressure and temperature conditions, the proppant does not agglomerate. Such precured resin coated proppants are typically held in the well by the stress surrounding them. In some hydraulic fracturing circumstances, the precured proppants in the well would flow back from the fracture, especially during clean up or production in oil and gas wells. Some of the proppant can be transported out of the fractured zones and into the well bore by fluids produced from the well. This transportation is known as flow back.
Flow back of proppant from the fracture is undesirable and has been controlled to a large extent by the use of a proppant coated with a curable resin which will consolidate and cure underground. Phenolic resin coated proppants have been commercially available for some time and used for this purpose. Thus, resin-coated curable proppants may be employed to “cap” the fractures to prevent such flow back. The resin coating of the curable proppants is only-partially crosslinked or cured before injection into the oil or gas well. The coating is designed to crosslink under the stress and temperature conditions existing in the well formation. This causes the proppant particles to bond together forming a 3-dimensional matrix and preventing proppant flow back.
Over the years, numerous improvements have been made to proppants in order increase their strength and stability while at the same time maintaining permeability at the required subterranean depths and pressures.
U.S. Pat. No. 3,492,147 to Young, et al., describes a method of coating particulate solids with an infusible resin. The particulates to be coated include sand, nut shells, glass beads, and aluminum pellets. The resins used include urea-aldehyde resins, phenol-aldehyde resins, epoxy resins, furfuryl alcohol resins, and polyester or alkyl resins.
Graham et al., in U.S. Pat. No. 3,929,191 describe particles coated with solid, fusible resin for use in treating subterranean formations. As described therein, particles such as glass beads are coated with a thermosetting resin such as one-step phenolics that, at formation conditions, first melts or softens, and then cure to form an insoluble, infusible cross-linked particle, allowing the particles to bond together and form a self-sustaining structure in the formation with a high compressive strength.
U.S. Pat. No. 5,643,669 to Tsuei has suggested a low volatile organic compound curable water-based particle coating composition, wherein the composition includes a urethane/acrylic copolymer having a glass transition temperature, Tg, of greater than 50° C. The coating composition further includes a polyoxyethylene aryl ether plasticizer, an alkaline-stable cross-linker, and water, and does not include undesirable amount of volatile organic coalescing agents.
In U.S. Pat. No. 5,916,933 issued to Johnson, et al., proppants comprising a particle coated with a composition containing bisphenol-aldehyde novolak resin, a bisphenol homopolymer, or mixtures of such polymers are suggested. Also offered are methods of making and using such proppant particles in subterranean formations, or in foundries. The resin composition also includes the incorporation of certain crosslinking agents, such as hexamethylenetetramine. The bisphenol-homopolymer-coated particles are further described as having a crush-strength comparable to that of conventional phenol-formaldehyde novolak polymer coated particulate material, but with an advantage of eliminated release of free phenol to the environment.
U.S. Pat. No. 6,059,034 to Rickards, et al. (issued May 9, 2000), describes a blend of fracture proppant material and deformable particulate material for fracturing processes. The deformable particulate material, such as polystyrene divinylbenzene beads, combines with the fracture proppant material, such as sand, to reportedly increase the fracture conductivity while simultaneously reducing fines generation and proppant flowback.
In U.S. Pat. No. 6,328,105, Betzold suggests a coated proppant and method of using such particles in increasing the conductivity and productivity of subterranean fractures. As described therein, the proposed proppant comprises a mixture of bondable particles and removable particles. The bondable particles, when in place in a subterranean formation, allegedly adhere to one another to form a self-supporting matrix that is interspersed with removable particles.
Finally, a composite proppant made of resin and a filler material for use in the fracturing of subterranean formations, as well as making the composite particles, is offered by McDaniel et al. in U.S. Pat. No. 6,406,789 (issued Jun. 18, 2002). The composite particles are described as being proppants made from fillers such as finely divided particles.
This multitude of approaches to resin-coated proppants, while varying in the nature of their components and processes for manufacture, all maintain a common feature—that is, they rely upon the formation of consecutive “layers” of resin coatings over the entire surface of the particular proppant. This approach is generally illustrated in FIG. 1, wherein the inner particle 2 has a generally spherical outer coating 4. Such a coating may be expanded to include other coatings, in which case there would be inner and outer coatings. Note that the coating thickness as shown in the illustration has been exaggerated for the purpose of clarification.
While a variety of useful proppants are known, there still remains the need for proppants having improved features such as high compressive strength, long term conductivity, i.e. permeability at the high closure stresses present in the subterranean formation, reduced fines production, bondability between proppant particles in the downhole environment, and improved flowback characteristics.